Zinc-plating compositions

ABSTRACT

A NOVEL ZINC-PLATING COMPOSITION COMPRISING ZINC POWDER AND A FLUX CONSISTING OF A FORMAMIDE AND/OR ACETAMIDE MEDIUM AND ZINC CHLORIDE, AMMONIUM CHLORIDE, STANNOUS CHLORIDE AND LEAD CHLORIDE IS PROVIDED. THIS COMPOSITION GIVES GOOD ZINC PLATING ON THE SURFACE OF STEEL MATERIALS BY SIMPLY BEING APPLIED TO THE SURFACE AND HEATED. UP TO 1% ALUMOINUM MAY BE ADDED TO SAID ZINC POWDER. ALUMINUM PREVENTS GROWTH OF THE ZINC-ION ALLOY LAYER. ALSO A PRIMARY FLUX SOLUTION CONSISTING OF ZINC CHLORIDE, AMMONIUM CHLORIDE, STANNOUS CHLORIDE, HYDROCHLORIC ACID AND WATER IS PROVIDED, WHICH IS USED FOR PRETREATING THE SURFACE TO BE ZINC-PLATING FOR ACTIVATION THEREOF. USE OF THESE COMPOSITIONS MAKES POSSIBLE ZINC PLATING OF LARGE SIZE STEEL MATERIALS AND STRUCTURES INCLUING SHIP BOTTOMS.

United States Patent 3,723,160 ZINC-PLATING COMPOSITIONS Jun Tanaka, Toshihiko Taguchi, and Mikio Watanabe, Tokyo, Japan, assignors to Senju Metal Industry Co., Ltd., Tokyo, Japan Filed Oct. 7, 1970, Ser. No. 78,798 Claims priority, application Japan, Oct. 22, 1969, 44/133,875 Int. Cl. B44d l /34; (23c /00 U.S. Cl. 117-50 ABSTRACT OF THE DISCLOSURE A novel zinc-plating composition comprising zinc powder and a flux consisting of a formamide and/or acetamide medium and zinc chloride, ammonium chloride, stannous chloride and lead chloride is provided. This composition gives good zinc plating on the surface of steel materials by simply being applied to the surface and heated. Up to 1% aluminum may be added to said zinc powder. Aluminum prevents growth of the zinc-ion alloy layer. Also a primary flux solution consisting of zinc chloride, ammonium chloride, stannous chloride, hydrochloric acid and Water is provided, which is used for pretreating the surface to be zinc-plating for activation thereof. Use of these compositions makes possible zinc plating of large size steel materials and structures including ship bottoms.

3 Claims BACKGROUND OF THE INVENTION This invention relates to a material for zinc plating. Iron and steel are tough materials but they have a disadvantage in that they are easily corroded. When iron and steel are plated with zinc which is a base metal, the zinc is corroded in preference to iron, and thus iron is protected. By way of utilizing this effect, zinc is the most widely and abundantly employed metal for coating iron materials for the purpose of inhibiting corrosion and rust formation. Zinked steel tubing, zinked iron wire and zinked steel plate or sheet are a few examples of such materials. That is, zinc is applied to the surface of iron materials for rust prevention by means of hot dip plating, electrolytic plating, sheradizing or melt spray.

However, in the cases of hot dip plating and electrolytic plating, the dimensions of the bath are limited and therefore the dimensions of things to be plated are limited, too. Steel materials such as hoop can be continuously plated by the above two methods, but only in limited widths. In sheradizing, the dimensions of materials to be treated are still more limited. The melt spray method does not give fully satisfactory zinked surfaces.

Therefore the above-mentioned prior art cannot be regarded to be useful for zinc-plating:

(1) steel tubing and other steel materials of large size; (2) construction steel materials and constructions; 3) ship bottoms, etc.

For these large steel pieces, zinc-rich paint which contains a large amount of zinc powder is now being used. But zincrich paint has the following defects:

(1) though it contains zinc powder, the paint is inferior to zinc plating in anticorrosion effect;

( 2) the coated film easily scales off;

(3) it is not applicable to steel materials which are used at rather high temperatures;

(4) it needs drying treatment taking 30 minutes to 1 hour after coating, etc.

In order to overcome these defects of the methods and materials for zinc plating, a method of melt plating using a paste comprising powder of tin, lead or tinlead alloy (Terne alloy) etc. and a flux was proposed and "ice tried because of low melting and simplicity in treatment. But these plating metals are nobler than iron, and therefore if pin-holes exist in the plated surface they accelerate corrosion of the iron substrate. Also, zinc-plating compositions which can be simply applied to the surface of a substrate and heated to effect zinc plating have been proposed, for instance, Japanese patent publication No. 4,882/69. However, these are compositions comprising zinc powder directly dispersed in an aqueous solution of inorganic salts such as zinc chloride. As zinc powder is very active, it reacts with the inorganic salt flux solution and the composition often becomes useless before it is to be used. Also these compositions are generally like a mixture of water and sand, and it is difficult to uniformly apply these compositions to the surface of steel materials with a brush. To overcome this defect, use of a flux carrier material such as glycerine or diethylene glycol was thought of, but these carrier materials are carbonized at high plating temperatures, so in practice, were impossible to use.

The object of this invention is to overcome the abovementioned defect of the prior art and to provide a novel material and process to effect zinc plating on the surface of iron and steel in a short period.

SUMMARY OF THE INVENTION In accordance with this invention, firstly, an easy method of zinc-plating iron and steel materials is provided which comprises applying by a brush or the like to the surface of steel and iron a composition comprising 30-80 parts by weight of a flux having the following formula:

Percent by weight Formamide, acetoamide or a mixture thereof 30-55 Zinc chloride 35-60 Ammonium chloride 6-12 Stannous chloride l-5 Lead chloride 1-7 Stannous chloride 1-10 Hydrochloric acid (sp. gr. 1.18) 1-3 Water Balance Total and preheating the substrate at a temperature between about 100 C. and about C. whereby vaporizing water in the primary flux and activating the surface of the steel or iron, and thereafter applying by a brush to the surface of iron or steel the abovementioned composition consisting of 30-80 parts by weight of a flux having the following formula:

Percent by weight Formamide, acetamide or a mixture thereof 30-55 Zinc chloride 35-60 Ammonium chloride 6-12 Stannous chloride 1-5 Lead chloride 1-7 Total 100 plus 20-70 parts by weight of zinc powder, powder of zinc alloy containing up to 1% aluminum, or zinc powder containing up to 1% aluminum powder, and thereafter heating the surface so as to provide the zinc coating as mentioned above.

Requirements for a zinc-plating material for melt plating are as follows:

(1) zinc powder which is incorporated in the plating material (composition) must not be dissolved or oxidized prior to being used;

(2) the plating material must be easily applied to the surface to be treated by a brush or the like;

(3) the plating material, which has been applied to the surface of iron or steel must not coagulate or dip down when heated;

(4) it must, of course, form a good plating; and so forth.

There is no zinc-plating material for melt plating which satisfactorily fulfil the above requirements among the hitherto known materials (Japanees patent publication No. 4,882/69, etc.). That is to say, in the development of effective methods for melt plating in which iron and steel are coated with a zinc-plating material and heated, if good plating function is to be secured, a large amount of very active ingredients must be incorporated in the flux, which results in difficulty in spreading said material by a brush and also in a very short shelf life. That is, zinc powder reacts with such active ingredients and is oxidized within several days. If the amount of the active ingredients is reduced, the amounts of the solvent (mainly water) is reciprocally increased. This improves spreadability but results in the following disadvantages. That is, such a rather thin plating composition, when heated after applied to the surface, bubbles as the solvent vaporizes, and most of the coated material drips down. If it does not drip down, as the flux is not so active, zinc powder is oxidized prior to being molten, and therefore does not give satisfactory plating. Because of these facts, it is extremely difiicult to provide uniform zinc plating on the surface of iron and steel by applying only a conventional zinc plating material.

Therefore, the first and most important feature of this invention is that an organic compound having polarity is used as the medium of the plating composition. In the group of the organic compounds which are miscible with metallic salts and protect metal powder from oxidation are included formamide, acetamide, dimethyl formamide, urea, guanidine, etc. But the compounds other than formamide, acetamide and a mixture thereof are not suitable for the reason that is explained later.

Formamide, acetamide and the mixture thereof are good solvents for the abovementioned metallic salts and protect the powder of zinc, zinc-aluminum alloy or mixed powder of zinc and aluminum which is in contact with said active metallic salts, and thus form a paste together with said salts and the metal powders which is easily coated and preserved for a long period. Also these organic compounds are less carbonizable when heated for plating.

When formamide and/or acetamide is used in an amount more than 55% of the total weight of the flux composition, the composition becomes liquid rather than paste, and therefore the coating property becomes poor. When less than 30% is used, the composition comes close to being solid, and the coating operation becomes difficult. Therefore it is suitable to use these organic compounds in an amount of 3055%. Acetamide is solid at room temperature, but when it is mixed with the salts in the molten state, it forms a paste together with the salts and remains in the paste state even after it is cooled. Use of less than 30% or more than 55% makes the composition close to being solid. By mixing formamide and acetamide in appropriate proportion, the consistency of the composition can be controlled. There is no restriction on the mixing ratio of formamide and acetamide in carrying out this invention.

Secondly, there is a specific feature in the formula of the flux of the plating material of this invention. As the flux in o 1 usual h t dip Zinc plating; a eutectic mixture (zinc chloride/ammonium chloride 3:1 by weight) is employed. But we have found that the flux of the plating composition of this invention is best in fluidity and plating effect when the ratio of zinc chloride to ammonium chloride is 5:1 by weight. The upper limit of the zinc chloride content is about 60% for the reason that when more than 60% zinc chloride is used, the plating composition which is formed together with formamide and/ or acetamide is close to being solid, and therefore the coating operation is difficult and the preservability is poor. The lower limit is about 35% because with less than 35% zinc chloride the composition is close to being liquid and thus it has a tendency to drip down when coated on the surface. The ammonium chloride content is one fifth of the zinc chloride content mentioned above, that is, about 6 to about 12% by weight.

The purpose of incorporating stanuous chloride is to improve the plating effect of the composition. Addition of stannous chloride improves the plating effect, but if the content thereof exceeds 5%, it tends to oxidize the zinc powder. It is necessary to add at least 1% stannous chloride to exhibit its effects. That is, the stannous chloride content should be 15% by weight.

The purpose of incorporating lead chloride is to improve the plating effect of the composition in combination with stannous chloride. The effect of lead chloride is especially to improve fluidity of the molten zinc. At least 1% is necessary in order to exhibit this effect. If more than 7% lead chloride is added, the composition becomes thick and stiff; and yet the plating effect does not improve in proportion with the content. Therefore the lead chloride content should be 1-7%.

The flux of the abovementioned composition is mixed with zinc powder, powder of zinc-aluminum alloy or mixed powder of zinc and aluminum (in both cases the aluminum content is up to 1%) to form a zinc plating paste. In order to keep the plating composition in a paste form, at least 30 parts by weight of the flux is necessary. Accordingly the metal powder occupies 70 parts by Weight at most. If the flux is less than 30 parts by weight, the plating composition becomes close to being solid, and the coating operation is made difiicult. If the flux occupies more than parts by weight (accordingly the metal powder content is less than 20 parts by weight), the plating composition drips down from the coated surface because of the excess amount of the flux when heated for plating. Therefore the preferred ratio of the flux and the metal powder is 30-80z70-20 by weight.

The reason why aluminum is added to zinc powder is that aluminum is effective to prevent growth of the interface layer (zinc-iron alloy layer) formed between the iron or steel substrate and the coated zinc layer. If said interface layer grows thick, the plated layer easily scales off when bending stress is applied, since the alloy is brittle. Aluminum has also an effect to inhibt the reaction between the metal powder and the flux ingredients. The upper limit of the aluminum content is 1%, since addition of more than 1% aluminum is not only ineffective for preventing said reaction but accelerating oxidation of the metal powder.

There is substantially no limitation on the particle size of the used powder of zinc, zinc-alloy and aluminum. If the particles are too coarse, coating and melt-plating operation becomes difficult. If the particles are too fine, chemical reactivty of the metal powder is too great and thus stability of the platng composition is lost. Normally, comercially available powder of -200 mesh gives satisfactory results.

The abovementioned zinc-plating composition is characterized by good preservability or long shelf life and excellent plating effect. But even better plating is effected by pre-treating the surface of the iron or steel to be plated with the primary flux composition. This is the third feature of this invention.

Prior to the use of the zinc-plating composition, the

primary flux is applied to the surface of iron or steel by means of a brush or the like and heated by a gas burner, a torch lamp or oxygen-acetylene flame, etc. When heated,

-the water in the flux vaporizes and the flux slightly effervesces. After the elfervescence, the non-volatile ingredients (corresponding to the flux part of the abovementioned plating composition) remain, and melt and dissolve rust and scale on the surface of the iron materials, and then turns brown. At this stage, heating is discontinued and the zinc-plating composition is applied.

The primary flux cleans and activates the surface of iron materials and also acts as the auxiliary flux for plating. Thus the use of primary flux improves the plating effect of the zinc-plating composition, prevents dripping of the coated plating composition, and inhibts oxidation of the metal powder which is caused when the flux is heated because of insufficiency of the active flux ingredients in the plating composition. (In the zinc-plating composition, a rather reserved amount the active metallic salt ingredients is used in order to prolong the shelf life.) That is the use of the primary flux doubles or triples the benefits brought about by plating with the zinc-plating composition of this invention.

The composition of the thus used primary flux is determined in conjunction with the composition of the flux part of the zinc-plating composition, and therefore it is a little different from that of the normally used flux for hot dip galvanizng. That is, the primary flux composition must contain as much amount of the non-volatile ingredient such as zinc chloride as possible, and must be miscible with the zinc plating composition which is later applied to the surface to be plated and thus must function as the plating flux. The primary flux is, therefore, comprised of 5075% by weight of zinc chloride, up to by weight of ammonium chloride, 1-10% by weight of stannous chloride, 1-3% by weight of hydrochloric acid (sp. gr. 1.18) the balance being water. The amount of zinc chloride varies correlatively with the amount of ammonium chloride, but if the zinc chloride content is less 50%, the water content is relatively high and thus the flux does not leave a suflicient amount of active salts after the water is vaporized. If the zinc chloride content exceeds 75% the flux is nolonger a solution. Consequently, the preferred zinc chloride contentis 50-70%. Ammonium chloride is added in order to improve the fluidity of the molten zinc chloride during plating. When 7075% zinc chloride is used, ammonium chloride does no more dissolve in the flux solution. In this case, the fluidity of the zinc chloride is improved by addition of stannous chloride, and therefore ammonium chloride is not necessary. When the proportion of zinc chloride is 50-70%, ammonium chloride is added. As the solubility of ammonium chloride in water is rather limited, it is not desirable to use more than 10% ammonium chloride. Stannous chloride improves the fluidty of the molten zinc chloride by lowering the melting point of the zinc chloride. Further, when the primary flux is applied on the surface of an iron material, it dissolves the scale and at the same time tin is deposited and forms a coating layer which helps zinc plating because tin is nobler than iron. Therefore stannous chloride is an indispensable ingredient for the zinc-plating composition, but it has a tendency to oxidize or dissolve the metal powder and therefore it cannot be incorporated in the flux part of the zinc-plating composition in large amount. Therefore, it is a feature of this invention that a considerable portion of the indispensable stannous chloride is incorporated in the so-called primary flux and its content is limited in the plating composition so as to secure a prolonged shelf life. Stannous chloride must be contained at least in an amount of 1%, use of more than 10% stannous chloride only increases the cost of the primary flux unnecessarily. Hydrochloric acid is added to stabilize the stannous chloride and prevent it from being converted to tin hydroxide. The amount of hydrochloric acid varies from 1 to 3% depending upon the amount of the added stannous chloride. A small amount of a surface active agent may be incorporated in the flux so as to improve coating and wetting properties of the flux, too.

After this primary flux is applied on the surface of the iron material and heated so as to remove water, the abovementioned zinc-plating composition is applied thereon and heated so as to effect zinc plating.

Now the invention is illustrated by way of working examples.

BRIEF EXPLANATION OF THE DRAWING FIG. 1 is a photomicrograph showing the cross section of a steel sheet zinc-plated with the zinc-plating composition of this invention.

FIG. 2 is a photomicrograph showing the cross section of a steel sheet zinc-plated by using the primary flux and zinc-plating composition of this invention.

FIG. 3 is a graph showing corrosion of pure zinc by the flux of the zinc-plating composition of this invention as the weight loss (in percent) as days pass in comparison with a prior art flux comprising an aqueous solution of inorganic salts.

DETAILED DESCRIPTION OF THE EMBODIMENT EXAMPLE 1 All the flux ingredients except for formamide shown in Table l were taken each in the indicated amount and molten and mixed in a vessel at about 250 C. After completely molten and mixed the mixture was cooled. To this indiacted amount of formamide was added and the mixture was heated at 170-180 C. so as to dissolve the solid salt. After the solids were completely dissolved, the mixture was rapidly cooled by being agitated. The thus obtained flux is in the state of a paste or ointment. The metal powder was mixed well into the paste and a zinc-plating composition was obtained.

This composition was uniformly applied with a brush on the sandpapered surface of a steel sheet (JIS 63310 SPCI) 0.8 mm. in thickness. After application, the applied composition was heated by sweeping the flame of a torch over the coated surface so as to vaporize the liquid component of the composition at -200 C. Then the flame was let stay longer in one place so that the zinc was molten and plated at 400-500 C. Thus the torch was slowly moved to melt the zinc powder portion by portion until the whole surface of the steel sheet was plated. The surface was cleaned with steel wool to remove the residual flux and the surplus molten zinc. The thickness of the zinc plating was about 20* microns, about 10 microns of which was an iron-zinc alloy layer. The test results are summarized in Table 1, too.

EXAMPLE 2 All the flux ingredients shown in Table 1 except for acetamide were taken each in the indicated amount and melted and mixed in a vessel at about 250 C. After completely molten and mixed the mixture was cooled. To this the indicated amount of acetamide was added and the mixture was heated at 170480 C. so as to dissolve the solid salts. After the solid salts were completely dissolved, the mixture was rapidly cooled as being agitated. The thus obtained flux is in the state of paste or ointment. The metal powder was mixed well into the flux and a zinc plating composition was obtained.

This composition was uniformly applied with a brush on the sandpapered surface of a steel plate (HS 63106 SM41B) 1-0 mm. in thickness. The applied composition was heated by an oxygen-acetylene flame so as to effect the zinc plating. The residual flux was washed away with water. The results are summarized in Table 1.

EXAMPLE 3 A zinc plating material having the composition indicated in Table 1 was prepared pursuant to the operation of Example 1.

This coomposition was applied to a steel tube (JIS C3454 STD G38) 3 mm. in thickness and 50 mm. in outer diameter, and zinc plating was effected by means of an oxygen-acetylene flame pursuant to the operation of Example 1. The plated sample was washed with water so as to remove the residual flux. The test results are summarized in Table 1.

EXAMPLE 4 All the flux ingredients shown in Table 1 except for acetamide and formamide were taken each in the indicated amount and melted and mixed in a vessel at about 250 C. After completely molten and mixed, the mixture was cooled. To this the indicated amount of acetamide and formamide were added and the mixture was heated at 170-1S0 C. so as to dissolve the solid salts. After the solid salts were completely dissolved, the mixture was cooled rapidly as being agitated. The thus obtained flux was in the state of paste or ointement. The metal powder was mixed well into the flux and a zinc plating composition was obtained.

Using this composition, a steel plate (JIS G33 08 SPMA) 1 mm. in thickness was zinc plated by means of a gas burner flame and the residual flux was washed away with water. The test results are summarized in Table 1.

EXAMPLE 5 A zinc plating material having the composition indicated in Table 1 was prepared pursuant to the operation of Example 1.

This composition was applied to a steel tube (JIS G3454 STP G38) 3 mm. in thickness and 50 mm. in outer diameter, and zinc plating was effected by means of the flame of a torch pursuant the operation of Example 1. The plated sample was washed with water so as to remove the residual flux. The test results are summarized in Table 1.

EXAMPLE 6 Primary flux composition All the primary flux ingredients shown in Table l were taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as the primary flux.

Zinc-plating composition The composition of Example 1 was used as the zinc plating composition.

The primary flux was applied with a brush on the sandpapered surface of a steel sheet (JIS G33l0 SPCl) 0.8 mm. in thickness. The surface was uniformly heated by the flame of a torch, so as to vaporize the water, whereby the steel surface was activated. When the flux turned slightly brown, heating was stopped and the zinc-plating composition was uniformly applied with a brush on the steel surface, which was heated evenly by sweeping the flame of a torch over said coated surface so as to vaporize the liquid component of the composition at 150- 200 C. Then the flame was let stay longer in one place so that the zinc was molten and plated at 400500 C. Thus the torch was slowly moved to melt the zinc portion by portion until the whole surface of the steel sheet was plated. The surface was cleaned with steel wool to remove the residual flux was surplus molten zinc. The thickness of the zinc plating was about 20 microns, about microns of which was an iron-zinc alloy layer. The test results are summarized in Table 1, too.

EXAMPLE 7 Primary flux composition All the primary flux ingredients shown in Table 1 were taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which is a primary flux. Zinc-plating composition The composition of Example 2 was used as the zincplating composition.

Using this composition, a steel plate (JIS G3106 SM41B) 10 mm. in thickness was zinc-plated by means of on oxygen-acetylene flame pursuant to the operation of Example 6, and the residual flux was washed away with water. The results were summarized in Table 1.

EXAMPLE 8 Primary flux composition All the primary flux ingredients shown in Table 1 were taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as the primary flux.

Zinc-plating composition The composition of Example 3 was used as the zincplating composition.

Using this composition, a steel tube (JIS C3454 STP G38) 3 mm. in thickness and 50 mm. in outer diameter was zinc-plated by means of an oxygen-acetylene flame pursuant to the operation of Example 6, and the residual flux was washed away with water. The test results are summarized in Table 1.

EXAMPLE 9 Primary flux composition All the primary flux ingredients shown in Table 1 were taken in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as primary flux to be used.

Zinc-plating composition The composition of Example 4 was used as the zincplating composition.

Using this composition, a steel plate (JIS G3308 SPMA) 1 mm. in thickness was zinc-plated by means of the gas burner flame pursuant to the operation of Example 6, and the residual flux was washed away with water. The test results are summarized in Table 1.

Comparative Example 1 All the flux ingredients of the zinc-plating composition shown in Table 1 except for dimethyl formamide were taken each in the indicated amount and were melted and mixed in a vessel at about 250 C. After completely molten the mixture was cooled. To this, the indicated amount of dimethyl formamide was added and heated at C. so as to dissolve the solid salts. After the solid salts were completely dissolved, the mixture was rapidly cooled as being agitated. The thus obtained flux is in an ointment-like state. The metal powder was mixed well into the flux and a zinc-plating composition was obtained.

Using this composition, a steel plate (J IS 63310 SPCl) 0.8 mm. in thickness was zinc-plated by means of the torch lamp flame pursuant to the operation of Example 1, and the residual flux was washed away with water. The test results are summarized in Table 1.

Comparative Example 2 All the flux ingredients shown in Table 1 except for urea were taken each in the indicated amount and were mixed and melted well in a vessel at about 250 C. To this, urea was added and the mixture was heated at 140- 150 C. and thereafter was rapidly cooled as being agitated. The metal powder was mixed well into the flux. The thus obtained plating composition was nearly in a solid state.

Using this composition, a steel plate (JIS G3'l06 SM41B) 10 mm. in thickness was zinc-plated by means of an oxygen-acetylene flame, and the residual flux was washed away with water. It was extremely difficult to apply this plating composition with a brush.

Comparative 'Example 3 All the flux ingredients shown in Table 1 except for formamide and guanidine were taken each in the indicated amount and were mixed and melted well in a vessel at about 250 C. After completely mixed, the mixture was cooled. To this formamide and guanidine were added and the mixture was heated at 140-150 C. so as to dissolve the solid salts. After the solid salts were completely dissolved the mixture was rapidly cooled as being agitated. To this, the metal powder was added and mixed well. The thus obtained plating composition is in a rather hard sherbet-like state.

Using this composition, a steel tube (JIS C3435 STP G38) 3 mm. in thickness and 50 mm. in outer diameter was zinc-plated by means of the oxygen acetylene flame pursuant to the operation of Example 1 and the residual flux was washed away. The test results are summarized in Table 1. It was rather difficult to apply this composition on a surface by a brush.

Comparative Example 4 All the flux ingredients shown in Table 1 except for urea and dimethyl formamide were taken each in the indicated amount and were melted and mixed in a vessel at about 250 C. After completely molten the mixture was cooled. To this, urea and formamide were added and the mixture was heated at 140150 C. so as to dissolve the solid salts. Thereafter, the mixture was rapidly cooled, and the metal powder was added and mixed well. The thus obtained zinc-plating composition was in the state of like a hard sherbet-like state.

Using this composition, a steel plate (HS 63308 SPMA) 1 mm. in thickness was zinc-plated by means of a gas burner flame pursuant to the operation of Example 1. and the residual flux was washed away with water. The results are summarized in Table 1. To apply this composition by a brush was possible but rather diflicult.

Comparative Example 5 Primary flux composition All the primary flux ingredients shown in Table 1 were taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as the primary flux.

Zinc-plating composition The composition of Comparative Example 1 was used as the zinc-plating composition.

A steel sheet (JIS 63310 SPCl) 0.2 mm. in thickness was zinc-plated by means of the torch flame pursuant to the operation of Example 6 and the residual flux was washed away with water. The results are summarized in Table 1.

Comparative Example 6 Primary flux composition All the primary flux ingredients shown in Table 1 were taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as the primary flux.

Zinc-plating composition The zinc-plating composition used in Comparative Example 2 was used.

A steel plate (JIS 63106 SM41B) mm. in thickness was zinc-plated by means of an oxygen-acetylene flame following the operation of Example 6 and the residual flux was washed away with water. The test results are summarized in Table 1. It was extremely difiicult to apply this composition by a brush.

Comparative Example 7 Primary flux composition All the flux primary ingredients shown in Table 1 were 10 taken each in the indicated amount and were mixed well in a vessel to give a clear solution, which was used as the primary flux.

Zinc-plating composition The zinc-plating composition used in Comparative Example 3 was used.

A steel tube (JIS C3454 STP G38) 3 mm. in thickness and 50 mm. outer surface was zinc-plated by an oxygen-acetylene flame following the operation of Example 6 and the residual flux was washed away with water. The results are summarized in Table 1. To apply this composition by a brush was possible but rather difiicult.

Comparative Example 8 Primary flux composition All the primary flux ingredients shown in Table l were taken each in the indicated amount and were mixed Well in a vessel to give a clear solution which was used as the primary flux.

Zinc-plating composition The plating composition used in Comparative Example 4 was used.

A steel sheet (JIS 63308 SPMA) 1 mm. in thickness was plated by means of the gas burner flame pursuant to the operation of Example 1. The residual flux was washed away with water. The test results are summarized in Table 1. To apply this plating composition by a brush was possible but very diflicult.

Reference Example 1 The zinc plating material having the composition shown in Table 1 (described in Japanese patent publication No. 4,882/69) was prepared in accordance with the explanation in said patent publication.

A steel sheet (JIS 63310 SPCl) 0.8 mm. in thickness was coated with said composition several times by means of a brush, and was heated with a torch flame so as to eifect zinc plating. After the sheet was plated, the residual flux was washed with water. The test results are summarized in Table 1. It was extremely difficult to apply this composition by a brush.

Reference Example 2 Another zinc-plating composition having the composition shown in Table 1 (described in said Japanese patent publication) was prepared in accordance with the explanation of said patent publication.

A steel tube 63454 STP 638) 3 mm. in thickness and 50 mm. in outer diameter was coated with said composition several times by a brush, and was heated with a torch flame so as to effect zinc plating. After the tube was plated, the residual flux was washed with water. The test results are summarized in Table 1. It was extremely difficult to apply this composition uniformly on the surface of the steel tube.

The figure shows test results of the study on the corrosiveness of fluxes of this invention and of the prior art. Corrosiveness against pure zinc (represented by weight loss in percent) of the flux of the zinc-plating composition of Examples 1 and 6 (curve B) and a prior art flux comprising 42.2% by weight zinc chloride, 9.1% by weight ammonium chloride, 1.6% by weight lead chloride, 0.3% by weight stannous chloride and balance water (curve F) is shown as the change as days pass. It is clear that the flux of the plating composition of this invention is extremely non-corrosive to zinc at the normal temperature and pressure.

As have been explained in detail, the method and composition for zinc plating of this invention makes possible rust-preventive zinc plating of large size steel materials, steel constructions, already constructed steel materials and ship bottoms, which have been regarded as impossible to 

